The present invention relates generally to composite electrical flow-through heaters useful for heating fluids, e.g., gases, for a variety of applications. More particularly, the invention relates to electrical heating elements and methods useful as, or in combination with, cellular or honeycomb structures of the kind commercially used for gas filters or catalytic converters. Such heaters are useful to initiate faster catalyst "light-off" in effluent treatment systems such as automotive catalytic converters, diesel particulate filters, wood stove converters, industrial stacks, and a variety of other systems in which an exhaust gas stream temperature is otherwise too low to initiate catalytic conversion reactions.
Pollutants from automotive exhaust systems, in particular, continue to present a substantial environmental challenge. Catalytic converters have been very successful in reducing automotive exhaust pollution, but as pollution standards become more stringent, there is an increasing need for better and improved converters.
The Federal Test Procedure (FTP) is an emission certification test used on light-duty vehicles. Cold-start under FTP standards is the starting of an engine after a period of 12 to 36 hours of non-operation in a temperature environment of 20.degree.-30.degree. C. For most cold starts, there is a time span of one to two minutes between the time the emissions begin and the time the catalyst and substrate heat up sufficiently to reach the light-off temperature of the catalyst. This time span is referred to as the light-off time; the light-off temperature is taken to be that temperature at which 50% conversion of the pollutants present in the treated gas stream is achieved. For automotive catalytic converters this temperature is typically in the 200.degree.-300.degree. C. range.
Some estimates are that about half of all pollutants escaping into the atmosphere from a vehicle equipped with a catalytic converter are generated in these first two minutes following cold start-up. Therefore, by reducing the light-off time, a very significant reduction in total pollutant emissions could be achieved.
Modifications to automotive and other catalytic converters to reduce the light-off time by preheating the converter or the gas to be treated have been proposed. U.S. Pat. No. 5,063,029, for example, suggests the use of an extruded metal honeycomb structure as a gas preheater or heated catalyst support for an automotive emissions control system. U.S. Pat. No. 3,163,841 discloses an electrical heater comprising a ceramic honeycomb structure provided with a conductive coating.
The supplemental heating of exhaust or feed gas streams is also of interest for certain gas filtration applications, a typical example being particulate filters for diesel exhaust streams. In these applications, particulate carbon is removed from the waste gas stream by filtration, with subsequent air oxidation of the collected particulates. U.S. Pat. Nos. 4,505,726 and 4,535,589 suggest the use of electrically heated metal or other conductive components, including metallic films, in order to achieve the gas or substrate temperatures necessary to oxidize these particulates.
For these and many other fluid heating applications a dimensionally stable heating element which can withstand hundreds of hours of high temperature use will be required. While not universally accepted, requirements for such heating elements may include any or all of the following: (i) a large geometric surface area for efficient heating of the gas stream, (ii) a low thermal mass to insure rapid heat-up, (iii) dependable operation at temperatures sufficient to achieve rapid gas heating into the 200.degree.-300.degree. C. preheat range, (iv) stability against oxidation and reduction at operating temperatures, and (v) thermal shock resistance and dimensional stability over the anticipated life of the element. Desirably, such elements should also be coatable by and non-poisoning with respect to one or more commercial catalysts, and should offer the capability of providing or supporting electrically insulating surfaces.
One of the disadvantages of utilizing an all-metal heating structure of the known cellular extruded type is the relatively large mass of metal needed to achieve structural and thermal integrity in the structure. The resulting high thermal mass means, at least for automotive applications, that a substantial amount of electric power is required to achieve effective preheating temperatures within the time interval (preferably less than 20 seconds and more preferably less than one second) considered acceptable by motorists. In addition, the resistance of extruded metal monoliths is somewhat difficult to control.
Wrapped corrugated steel or other metal preheater designs disadvantageously require the incorporation of electrically insulating coatings to avoid short circuiting, and the coatings are prone to loss or damage in use through vibration. In addition, at temperatures in the 700.degree. C. range and above, corrugation straightening as well as drops in elastic modulus and yield strength occur in such structures, compromising the dimensional stability and rigidity of the product.
Preheater systems based on the application of conductive films to rigid ceramic substrates, while offering the potential of lower thermal mass, must confront the problem of long term thermal durability. The adverse chemical and physical factors found in a typical thermally cycled exhaust environment simply accentuate the problem of maintaining physical integrity in composite systems incorporating high expansion metal films on rigid low expansion ceramics.
Accordingly, it is a principal object of the present invention to provide heating elements and fluid heating apparatus incorporating those elements which offer a combination of good heating efficiency and good thermal durability.
It is a further object of the invention to provide a gas preheater suitable for use in a wide variety of effluent heating applications including automotive emissions control, diesel fume treatment, wood stove smoke conversion, industrial stacks emission control or the like, wherein the exhaust gas stream temperature at start may be too low to initiate rapid catalyst light-off.
Other objects and advantages of the invention will become apparent from the following description thereof.